1. Field of the Invention
The present invention relates to an access point, wireless communication station, wireless communication system and wireless communication method.
2. Related Background Art
Recently wireless LAN interfaces are installed in various devices. In particular, the popularization of the IEEE 802.11 standard for personal computers, home electronic products and portable terminals is remarkable. IEEE 802.11 is a standard that specifies the wireless LAN technology for layer 2, particularly for the MAC or lower layers. When a portable terminal is used as a station based on IEEE 802.11, the technology to implement power saving is critical. In Non-patent Document 1, a following method to implement power saving in a station according to IEEE 802.11 is used. First an overview of an intermittent reception method in a station according to IEEE 802.11 will be described. In IEEE 802.11, two types of intermittent reception methods, that is an intermittent reception method for a unicast packet, and an intermittent reception method for a broadcast/multicast packet, are specified.
FIG. 1 shows the intermittent reception method for a unicast packet. FIG. 1 is a timing chart depicting a conventional packet transmission processing. As FIG. 1 shows, an access point AP1a periodically sends a beacon P1 to a wireless communication station STA1a. The wireless communication station STA1a during intermittent reception wakes up (turns ON the wireless LAN interface) immediately before a beacon transmission schedule time, and receives the beacon P1. In the beacon P1, “0” or “1” data is set for each AID (Association ID). This data indicates whether a unicast packet P2 corresponding to each AID is buffered in an access point AP1a or not. The AID and “0” or “1” data, which is set for each AID, are called a “bit map”. Here the AID is a characteristic value which the access point AP1a allocates to a wireless communication station STA1a when a wireless communication station STA1a is connected to the access point AP1a, and a value in the “1” to “2007” range is used. In other words, a bit for AID includes the 1st to 2007th bit. The wireless communication station STA1a, which receives the beacon P1, maintains the Awake state (the wireless LAN interface conforming to IEEE 802.11 standard is in the ON state, and communication operation is in the drive state) if the received beacon P1 is referred to and “1” is set in the bit for AID allocated to the local station (wireless communication station STA1a), and sends PS-Poll, which is a packet to request transfer of the unicast packet P2, to the access point AP1a. 
The period when the wireless communication station STA1a is in the Awake state is shown by the symbol T1 in FIG. 1. The period when the wireless communication station STA1a is in the Doze state (the wireless LAN interface conforming to IEEE 802.11 is in the OFF state and the communication operation is in the pause state) is a period other than the periods indicated by the symbol T1 in FIG. 1. The access point AP1a, which received the PS-Poll, returns Ack1 (Ack: acknowledge) to the wireless communication station STA1a, which is the transmission source of the PS-Poll, and sends a buffered unicast packet P2 to the wireless communication station STA1a. The wireless communication station STA1a receives the unicast packet P2 from the access point AP1a, and returns Ack2 to AP1 after reception, and transits to the Doze state. On the other hand, the wireless communication station STA1a which received the beacon P1 immediately transits to the Doze state if the received beacon P1 is referred to and “0” is set in the bit for AID allocated to the local station (wireless communication station STA1a). By the above operation, the unicast packets are intermittently received.
Now the intermittent reception method for a broadcast/multicast packet (means a broadcast packet or a multicast packet, the same herein below) specified in IEEE 802.11, will be described with reference to FIG. 2. FIG. 2 is a timing chart depicting a conventional packet transmission processing. As FIG. 2 shows, an access point AP1a periodically sends a DTIM beacon P3 (DTIM: Delivery Traffic Indication Message) to a wireless communication station STA2. The DTIM beacon P3 is a special beacon which is sent for a predetermined number of beacons. Therefore the transmission cycle of the DTIM beacon P3 is a beacon cycle multiplied by a natural number. Normal beacons are omitted in FIG. 2. The wireless communication station STA2 during intermittent reception wakes up immediately before the transmission schedule time of the DTIM beacon P3, and receives the DTIM beacon P3. The DTIM beacon P3 includes the above mentioned bit map for AID, which is used for notifying a unicast packet buffering, just like a normal beacon, and additionally includes a No. 0 bit for AID.
The No. 0 bit for AID of the DTIM beacon P3 is created for notifying the buffering of a broadcast/multicast packet. In the No. 0 bit for AID of the DTIM beacon P3, “1” is set if the broadcast/multicast packet P4 is buffered in the access point AP1a, or “0” is set if the broadcast/multicast packet P4 is not buffered in the access point AP1a. The wireless communication station STA2, which received the DTIM beacon P3, maintains the Awake state if “1” is set in the No. 0 bit for AID, and waits for the transmission of a broadcast/multicast packet P4 from the access point AP1a. 
The broadcast/multicast packet P4, which is transmitted from the access point AP1a, consists of one or more packets. In the last packet, out of one or more packets constituting the broadcast/multicast packet P4, data to indicate this information (that is, this is the last packet, and there is no more buffered broadcast/multicast packet P4 hereafter) is set. When the last packet out of one or a plurality of packets constituting the broadcast/multicast packet P4 is received, the wireless communication station STA2 transits to the Doze state. Ack in the MAC layer according to IEEE 802.11 is transmitted/received when a unicast packet is transmitted/received, but is not transmitted/received when a broadcast/multicast packet is transmitted/received. The wireless communication station STA2 which received the DTIM beacon P3 immediately transits to the Doze state if “0” is set in the No. 0 bit for AID.    [Non-patent Document 1] ANSI/IEEE Std 802.11, 1999 Edition (R2003) Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications